Technical Field
The present invention relates generally to electronic devices and, in particular, to driving the common-mode of a Josephson parametric converter using a three-port power divider.
Description of the Related Art
A Josephson ring modulator (JRM) is a nonlinear dispersive element based on Josephson tunnel junctions that can perform three-wave mixing of microwave signals at the quantum limit. The JRM consists of Josephson Junctions (JJs). In order to construct a non-degenerate parametric device that is the Josephson parametric converter (JPC), which is capable of amplifying and/or mixing microwave signals at the quantum limit, the JRM is coupled to two different microwave resonators.
In microstrip JPCs, as well as compact and shunted JPCs, the pump drive which provides the energy for the amplification process is fed through the sum port (Σ) of a 180 degree hybrid coupler. The difference port (Δ) of the same hybrid is used to feed the differential modes signal or idler tones to the JPC. In this configuration, both the pump and the signal or idler are fed to the JPC through the same feedlines and coupling capacitors of the JPC (to which the 180 degree hybrid coupler is connected).
In amplification, the pump frequency is at the sum of the idler and signal frequencies and since the idler and signal frequencies are usually in the 4-15 Gigahertz range, the pump frequency is typically several Gigahertz apart from the signal and idler frequencies. Thus, in order to feed both the pump and signal or idler tones through the same hybrid, the hybrid needs to be broadband enough to accommodate those two different frequencies. For that purpose, existing JPCs use commercial broadband hybrids which are big in size and are off chip. The addition of this bulky hardware limits scalability. To solve this problem, one can design and implement broadband hybrids on-chip but this would add complexity to the design and fabrication processes. For example, if we couple two JPCs on the same chip to form a quantum-limited Josephson directional amplifier, placing the hybrids in plane would require wire cross-overs.
Feeding the pump and the signal (or idler) tones to the JPC through the same coupling capacitors poses a tradeoff between the device bandwidth and dynamic range. By increasing the coupling capacitors of the resonators, the device bandwidth increases since the coupling to the feedline (i.e., external circuit) increases, but it also increases the coupling to the second harmonic resonance of the microstrip resonator which the pump tone (the common mode) is close in frequency to. This softens the pump drive (makes it less “stiff”) and consequently decreases the dynamic range of the JPC.